1. Field of the Invention
This invention relates to pneumatic radial tires, and more particularly to a pneumatic heavy duty radial tire for truck and bus use under a higher internal pressure which avoids separation failure at the belt end by specifying the arrangement of cords constituting the belt.
2. Description of the Related Art
JP-Y-48-96259 discloses a truck and bus type tire wherein a belt having no cut surface of cord at its end is formed by bending cords slantly extended with respect to an equatorial plane of the tire at a widthwise end of the belt to zigzag extend in a circumferential direction of the tire for avoiding the occurrence of separation failure at the belt end.
In such a belt, the cords are crossed with each other at the widthwise end of the belt and are continuously extended at an axially bent state, so that no cord shifting occurs between upper and lower belt layers at the belt end. Hence the stretching of the belt end in the circumferential direction is very small as compared with the conventional belt structure where rubberized cord plies having a given width are laminated one upon the other so as to cross the cords of the upper and lower plies with each other. This is particularly true when in case of using high modulus cords such as steel cords and the like are used.
On the other hand, a distance or a diameter from a rotational axis of the tire to an outer surface of a tread is small in an widthwise end of the tread than in a widthwise central portion of the tread, i.e. there is a difference in diameter between tread end and tread center. As a result, a shearing deformation in the circumferential direction is apt to be caused at the widthwise end of the tread locating within a ground contact area during running under load in the case of heavy duty pneumatic tires for truck and bus use having a large diameter difference. The shearing deformation creates a large slipping between tread end surface and road surface, and consequently brings about a so-called shoulder wear the tread end is prematurely worn compared with the other portion of the tread. For this end, the belt cord plies are laminated one upon the other so as to cross the cords of the upper and lower plies with each other, whereby the stretching of the belt end in the circumferential direction is ensured to absorb the diameter difference between the central portion of the tread in widthwise direction and the widthwise end thereof.
However, when the belt having a very small stretching in the circumferential direction at the belt end is used as mentioned above, since the stretching in the circumferential direction can not be expected at the belt end, it is difficult to absorb the diameter difference between the widthwise central portion and widthwise end of the tread and hence the occurrence of shoulder wear is not avoided. Such an occurrence of shoulder wear still remains a serious problem in the tire using the belt having no cut surface of cord at the belt end for avoiding the belt end separation.
As a means for controlling the shoulder wear, it has also considered to render the contour of the tread surface into a crown shape having a large radius of curvature for reducing the diameter difference between the central portion and end portion of the tread. In this case, however, the ground contact pressure rises at the tread end to increase the heat build-up and hence it is apt to be caused the separation failure at the belt end.